This invention relates to an engine exhaust purification device comprising a NOx trap catalyst.
JP-A-2000-54824 published by the Japanese Patent Office in 2000 discloses an engine wherein NOx (nitrogen oxides) in exhaust gas are trapped by the NOx trap catalyst when the air-fuel ratio in the catalyst is lean, and the trapped NOx is released from the NOx trap catalyst when the air-fuel ratio in the catalyst is the stoichiometric air-fuel ratio or a rich air-fuel ratio.
In an engine provided with this type of NOx trap catalyst, NOx discharged from the engine while the engine is running under a lean air-fuel ratio is trapped by the NOx trap catalyst so that discharge of NOx into the atmosphere is prevented. However, the NOx amount which can be trapped by the NOx trap catalyst is limited, and as the NOx trap rate decreases when the NOx amount increases, it is necessary to release and reduce NOx when the NOx trap amount increases.
Hence in general, when the trapped NOx amount increases, the fuel ratio of the engine is shifted to rich and the air-fuel ratio in the catalyst is controlled to stoichiometric or rich, and the NOx trapped in the NOx trap catalyst is released and reduced, for a predetermined time.
As described above, when NOx in the NOx trap catalyst is released and reduced, the air-fuel ratio of the engine may be shifted to rich, and a reducing agent may be supplied to the NOx trap catalyst.
However, if the amount of reducing agent supplied is small relative to the oxygen amount stored by the catalyst, the trapped NOx cannot be fully released and reduced. This is because even if the air-fuel ratio of the engine is shifted to rich, the air-fuel ratio in catalyst returns to lean to some extent due to the oxygen released from the catalyst, and the atmosphere in the catalyst shifts from the target stoichiometric air-fuel ratio or rich air-fuel ratio.
Conversely, if the amount of reducing agent supplied is large relative to the oxygen amount stored by the catalyst, the air-fuel ratio in the catalyst becomes richer than necessary, and the amount of CO and HC (hydrocarbons) discharged into the atmosphere increases.
It is therefore an object of this invention to release and reduce NOx with high efficiency by supplying a reducing agent which is neither in excess or deficiency when NOx trapped in the catalyst is released and reduced, and to prevent increase of CO and HC discharged into the atmosphere.
In order to achieve above object, this invention provides an exhaust gas purification device for an engine, comprising an exhaust passage of the engine, a first catalyst, installed in the exhaust passage, that stores and releases oxygen in exhaust gas in accordance with an air-fuel ratio of exhaust gas, a second catalyst, installed in the exhaust passage downstream of the first catalyst, that traps and releases NOx in exhaust gas in accordance with the air-fuel ratio of exhaust gas, and a microprocessor programmed to perform a rich operation that controls the air-fuel ratio of exhaust gas to rich when trapped NOx in the second catalyst is to be released, wherein an amount of reducing agent in exhaust gas due to the rich operation is increased with increase in a total oxygen amount stored in the first catalyst and the second catalyst.
According to an aspect of this invention, this invention provides a method for purifying exhaust gas discharged from an engine, comprising computing a total oxygen amount stored in a first catalyst and a second catalyst based on an engine operation condition, wherein the second catalyst being disposed downstream of the first catalyst, performing a rich operation that controls the air-fuel ratio of exhaust gas to rich when trapped NOx in the second catalyst is to be released, wherein an amount of reducing agent in exhaust gas due to the rich operation is increased with increase in a total oxygen amount stored in the first catalyst and the second catalyst.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.